Increase the client_request size to 50m

We are hitting the maximum request size of 30m when running imports on
the site-activity dashboard for top content. At some point we will need
to look at making the client request size smaller - this is a temporary
fix so we can collect data now.
Merge pull request #432 from alphagov/increase-client-request-limit

Increase the client_request size to 50m